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Graphene Optical Switches One Hundred Times Faster Than Current Devices

Just two years ago, Andre Geim and Konstantin Novoselov, the two scientists who won the Nobel Prize for discovering graphene, succeeded in improving photodetectors to the degree that they could boost optoelectronic data transfer rates by a factor of 20. That breakthrough relied on using graphene combined with plasmonic nanostructures.

Now researchers at the University of Bath in the U.K. are reporting measurements indicating that graphene could lead to optical switches that are nearly a hundred times faster than materials used in today’s current switches.

The research, which was published in the journal Physical Review Letters (“Carrier Lifetime in Exfoliated Few-Layer Graphene Determined from Intersubband Optical Transitions”), found that the response rate of an optical switch using graphene to be around 100 femtoseconds, which is about a hundred times faster than the few picoseconds measured in today’s devices.

“We’ve seen an ultrafast optical response rate, using few-layer graphene, which has exciting applications for the development of high speed optoelectronic components based on graphene,” said lead researcher Dr. Enrico Da Como in a press release. “This fast response is in the infrared part of the electromagnetic spectrum, where many applications in telecommunications, security, and also medicine are currently developing and affecting our society.”

In addition to photodetectors and optical switches, graphene is proving attractive for tunable notch filters, an area where IBM has made some interesting progress. Also, researchers have been able to exploit graphene’s wide spectral range for different kinds of tunable lasers that are used in optoelectronic systems.

In fact, the research team’s long-range goal is to apply this discovery to the development of graphene-based quantum cascade lasers that could be used for pollution monitoring, security, and spectroscopy applications.

Image: Martin McCarthy/iStockphoto

Graphene Protects Metal Silicides From Oxidation

While much research into graphene for electronics applications has focused on ways to have it replace silicon, a research group at the University of Vienna is looking at ways to integrate graphene into current silicon-based technologies.

The Vienna researchers along with colleagues in Germany and in Russia took an approach to integrating silicon within graphene that involved building a semiconducting or an insulating buffer between graphene and a metallic substrate.

With this aim, the international team have successfully built a structure of high-quality metal silicides covered and protected underneath a graphene layer. Metal silicides, which are a compound of silicon with a more electropositive element, are used extensively in applications including complementary metal oxide semiconductor (CMOS) devices, thin film coatings and photovoltaics as interconnects and barriers.

The research published in Nature’s new open-access journal Scientific Reports (“Controlled assembly of graphene-capped nickel, cobalt and iron silicides”) used monocrystalline layers of films of nickel, cobalt and iron as the substrate on top of which high-quality graphene produced through chemical vapor deposition (CVD) was deposited. The resulting structure is protected against oxidation because of the graphene capping the metal silicide layers.

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Seven or Never: On-Off Adhesives Make Progress Towards Commercial Applications

Sometimes the biggest inspiration for technological development is nature. Biomimicry—as it is known—is leading to advancements in wind power where the small bumps on the leading edges of the fins of humpback whales are duplicated on the blades of wind turbines.  Another example of this biomimicry is the tiny hairs on the petals of the lotus flower that repel water being mimicked in the waterproof coatings for mobile phones.

In the field of nanotechnology and biomimicry there has been one creature of nature that has been of particular interest: The gecko. Researchers have been fascinated with the gecko’s gravity-defying ability to walk on ceilings and not fall. The gecko’s exploits are accomplished by hundreds of thousands of tiny hairs, called setae, covering their feet. Each one of these setae itself has nanoscale projections, which are so small that they produce the weak molecular interactions known as van der Waals between themselves and the substrate.

Over three years ago, I covered research coming out of the University of California Santa Barbara (UCSB) that was exploiting the gecko’s design to potentially produce industrial adhesives that could be actuated—or turned on and off, like a switch—by magnetism.  I joked at the time that it could enable Spiderman-like super powers. But I also suggested that this was a technology that was commercially attractive.

With this commercial opportunity suggested by none other than myself, I thought I should investigate how far along the technology had developed as part of the ongoing series: “Seven or Never,” which looks back at technologies I have covered over the years to check on the current state of development.

Professor Kimberly L. Turner, who has been leading this research for the last decade at UCSB, gave me the update.

“We began working on this research in about 2003,” Turner told me via email. “My student at the time, Michael Northen, got interested in synthetic adhesives, and we were the first group to really focus on the hierarchical nature of the problem by using active MEMS technology.  We were able to integrate meso-, micro-, and nano- scales into an active device that could be changed from an adhesive state to a non-adhesive state.  Later on, we were able to use magnetic actuation to 'release' the adhesives from a stuck state.  It was a very exciting time.  That was back in 2006.  The work has grown in many directions since that early work.”

In addition to Northen, Turner has worked with three other PhD candidates in this area, including Abhishek Srivastava, Sathya Chary, and John Tamelier.

The work of Tamelier has been aimed at scaling up the adhesive patches that the team has been developing. He is also designing and building a test mechanism to study the best ways to approach surfaces with the adhesives in order to maximize adhesion.

“John Tamelier is about to have a paper come out in Langmuir which focuses on that work,” said Turner. “This is an essential result for achieving high adhesion with micro-robots.  How the foot of the robot approaches the surface it is running on is key to how much adhesion is generated.  The adhesives we are using for this are passive adhesives, meaning that they work without actuation.  However, they are anisotropic, meaning if moved in one direction they are sticky, but in another they are not sticky, thus creating more flexibility and ease in removal.”

Turner is quick to mention that the work in active devices (those that can be actuated to turn on or off) that were mentioned in my initial blog post covering their technology has continued.

“In a collaboration with the Army Research Lab and a local MEMS foundry, IMT Inc., we integrated thermal actuators and piezoelectric actuators into the adhesives,” explained Turner in an e-mail. “We were able to show that by adding force (from the thermal actuators) in the direction perpendicular to the pull-off direction, we could enhance the adhesion.  This was a very challenging fabrication, and was limited as to how much surface area we could generate, but it is a promising result.”

The aim of the “Seven or Never” series is to find out how far along a hopeful technology gets over a period of time. While I first covered the technology just three years ago, the first successful results were achieved back in 2006—right in line with our seven-year time frame. But as we learned from our first installment of this series, seven years is hardly enough time to bring an emerging technology to market and this one appears to be no exception. Nonetheless, the researchers are pursuing real-world, commercial applications.

“We do hold a patent on the magnetic technology, and have had quite a lot of interest from industry on commercializing,” said Turner.  “We have not been focused on that as of late, as it was not quite to the stage where it is useful for many of the applications of interest, but it is close.  We are excited about the future of this technology.”

With efforts being made in commercializing their technology, I asked Turner what her thoughts were on the challenges facing innovation and bringing an emerging technology to market.

“Easier industrial partnerships would certainly make this a lot smoother,” said Turner. “ There always seems to be a problem with intellectual property agreements, and this takes a lot of time and effort to iron out.  If there was an easier way to get this done, I think more industry would partner with universities, and it would lead to faster innovation.”

Despite these challenges, Turner added: “I bet there will be some products in the next 5 years…the possibilities are vast.”

Photo: Canebisca/iStockphoto

Nanoparticle Ink Enables 3-D Printing of Microbattery Electrodes

Over two-and-a-half years ago, researchers at Sandia National Laboratories developed what they claimed was the smallest battery yet produced. The lithium-based battery they produced was no bigger than a grain of sand, so small that one of its anodes consisted of nothing more than a single nanowire.

There didn’t seem to be any real-world applications for the battery, because it was created inside a transmission electron microscope (TEM). Instead it was intended to demonstrate a way forward in the miniaturization of batteries to satisfy a market in which gadgets are becoming smaller and smaller but the batteries used to run them remain rather large.

Now researchers at Harvard University are following up on Sandia's battery miniaturization by using a 3-D inkjet printing process enabled by nanoparticles made from lithium metal. This research brings 3-D printing to a new level, according to the researchers.

“Not only did we demonstrate for the first time that we can 3D-print a battery; we demonstrated it in the most rigorous way,” said Jennifer A. Lewis a professor at the Harvard School of Engineering and Applied Sciences (SEAS), in a press release.

The research, which was published in Advanced Materials (“3D Printing of Interdigitated Li-Ion Microbattery Architectures”), updates the traditional method of building electrodes that involves depositing thin films of solid materials. On it's own, this long-used technique results in solid-state micro-batteries that don’t store enough energy for today’s devices.

Instead the process Lewis and her colleagues employed used 3-D printing to build tightly interlaced, ultrathin electrodes. To do this, the team had to develop a special type of ink that would be electrochemically active and harden into layers as narrow as those produced by the thin-film manufacturing methods. The researchers developed ink for the anodes made from one compound of lithium metal oxide nanoparticles and from a different compound for the cathodes.

After depositing the inks onto two gold combs (a process you can watch on the video below), the stacks of interlaced anodes and cathodes were packaged into a container filled with electrolyte. The researchers were impressed with the performance measurements of the resulting battery.

“The electrochemical performance is comparable to commercial batteries in terms of charge and discharge rate, cycle life, and energy densities,” said Shen Dillon, an assistant professor of materials science and engineering at Harvard and one of Lewis' collaborators, in the press release. “We’re just able to achieve this on a much smaller scale.”

If the miniature batteries can be produce on a bulk scale, this could change the way in which small devices are powered and open up entirely new possibilities for electronic devices in both medial and non-medical applications.

Image: Jennifer A. Lewis, Harvard University

2-D Nanomaterials Put Photovoltaics on a Diet

Soon after graphene was discovered back in 2004, a number of other two-dimensional (2-D) materials appeared, vying for the same attention. By now, the list has grown to a veritable catalogue of 2-D materials.

The story of these 2-D materials will ultimately be about how well they can adapt to different applications that we consider for them today, or may discover in the future. Of them, perhaps no area has been as hotly pursued as photovoltaics (PVs).

One interesting new potential application of graphene is in creating what is known as “hot carrier” cells in which the graphene produces, after absorbing one photon, is capable of generating multiple electrons instead of just a single one. But the main focus of applying graphene to PVs has been as a replacement for indium tin oxide (ITO) used in the electrodes of organic solar cells.

Now researchers at MIT are looking at 2-D materials, such as molybdenum disulfide and molybdenum diselenide, to make the thinnest and lightest PVs ever made. While this may not produce the highest energy conversion efficiency or be the cheapest material for PVs—the two typical metrics most sought after—they do expect that their lightness should create some possibilities in this application.

Graphene's own energy conversion capabilities are not what you would call impressive, and more generally, two-dimensional materials don’t really compete with the 18-19 percent conversion efficiencies of standard silicon cells already on the market. But in the research, which was published in the ACS journal Nano Letters (“Extraordinary Sunlight Absorption and 1 nm-Thick Photovoltaics using Two-Dimensional Monolayer Materials”),  the MIT team demonstrated that if you stacked just three sheets on top of each other, a 1nm-thick stack can absorb up to 10 percent of incident sunlight, which is one order of magnitude higher than gallium arsenide and silicon. While the actual conversion efficiency is still pretty low at 1 percent, it does correspond to power densities being 100 to 1000 times higher than the best existing ultrathin solar cells.

“Stacking a few layers could allow for higher efficiency, one that competes with other well-established solar cell technologies,” said Marco Bernardi, a postdoc in MIT’s Department of Materials Science, in a press release.

Jeffrey Grossman, Associate Professor of Power Engineering at MIT and the paper's senior author, added in the release: “It’s 20 to 50 times thinner than the thinnest solar cell that can be made today. You couldn’t make a solar cell any thinner.”

Whether thinner and lightness will really translate into something that the market demands remains to be seen. The problem, as the researchers seem to concede, is that at this point there is no way to produce molybdenum disulfide and molybdenum diselenide in bulk. Until manufacturing techniques are developed that make that possible—and economical—the thinnest solar cells will likely remain laboratory curiosities.

Illustration: Jeffrey Grossman and Marco Bernard/MIT News

Is Putin Pulling the Plug on Russian Nanotechnology?

Russia’s generously funded and much ballyhooed nanotechnology initiative, Rusnano, has had its share of intrigue and certainly many detractors since its launch, not the least of which have been the leaders of the government, such as former president Dmitry Medvedev. But still it managed to continue on and seemed to be tracking fairly well with reported revenues of $300 million for 2011.

Just when it seemed Russia had found a shortcut into the nanotechnology arms race that has developed over the last decade and was sweeping up all the discarded nanotechnology companies that had run aground on the rocks of capitalism, Russian President Vladimir Putin last month looked to be sacrificing both Rusnano and another technology project Skolkovo—an attempt to build a Silicon Valley outside of Moscow—to  solidify his political aims.

As reported in last month’s Bloomberg, Putin was coming down hard on these two technology initiatives to project that he was tough on corruption and mismanagement of public funds.

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Nanomaterials Go Beyond Post-Silicon to Post-Semiconductor

Yesterday, IEEE Spectrum published a feature “Changing the Transistor Channel” that chronicles the laborious migration from the ubiquitous silicon in transistors to new materials, primarily compound semiconductors known as III-Vs.

These efforts to replace the semiconducting silicon in the channels of transistors is being pursued by all the big chip manufacturers and international research labs.. Various nanomaterials from graphene to nanowires made from III-V materials are being experimented with to help achieve that aim.

As momentum builds in this field, researchers at Michigan Technological University (MTU) are looking ahead not only beyond silicon but also to when semiconductors will not even be needed for transistors.

Yoke Khin Yap, a physicist at MTU, and his colleagues, including those at Oak Ridge National Laboratory (ORNL), have developed a method by which they use an insulator—boron nitride nanotubes—coupled with quantum dots to create a path for electrons to travel between electrodes in a transistor. No semiconductor material is used in the design.

“The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top,” Yap said in a press release. “In principle, you could get a piece of plastic and spread a handful of metal powders on top to make the devices, if you do it right. But we were trying to create it in nanoscale, so we chose a nanoscale insulator, boron nitride nanotubes, or BNNTs for the substrate.”

Two years ago, Yap and his team developed a way to make a virtual carpet out of BNNTs. In this latest research, which was published in the journal Advanced Materials (“Room Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots”),  the MTU team devised a method for depositing gold quantum dots on the BNNT carpet using a laser. The BNNTs turn out to be perfect for the job. They have controllable and uniform diameters so they can confine the size of the quantum dots.

When Yap and his colleagues, along with scientists at ORNL, put a voltage on the electrodes, they observed that the electrons jumped from one gold quantum dot to the next in an orderly fashion. This phenomenon is known as quantum tunneling. One benefit of this device is that the quantum tunneling effect is achieved at room temperature conditions.

“Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river,” said Yap in a press release. “The electrons hopped between the gold stepping stones. The stones are so small, you can only get one electron on the stone at a time. Every electron is passing the same way, so the device is always stable.”

This design allowed for the creation of a transistor that did not require a semiconductor. When sufficient charge was applied, the material was in a conducting state. When the charge was removed, it reverted back to being an insulator. An additional benefit to the design was that it didn’t suffer any “leakage” of electrons that plagues silicon, creating overheating problems and wasted energy.

Yap notes: “Theoretically, these tunneling channels can be miniaturized into virtually zero dimension when the distance between electrodes is reduced to a small fraction of a micron.”

Image: Yoke Khin Yap

Carbon Nanotubes Capture Electrical Signals Between Neurons

President Obama’s BRAIN initiative, which was launched back in April, may already have a new tool for mapping the human brain in its arsenal . Researchers at Duke University have used a carbon nanotube to capture electrical signals from individual neurons.

With a complete 3-D digital map of the human brain now available as part of the European Human Brain Project, brain research is gaining a lot of momentum. The carbon nanotube probe developed by the Duke team, which acts like a sort of harpoon, first spearing the neurons and then collecting the electrical signals they send to communicate with other neurons, is expected to provide a new level of insight into the human brain.

“To our knowledge, this is the first time scientists have used carbon nanotubes to record signals from individual neurons, what we call intracellular recordings, in brain slices or intact brains of vertebrates," said Bruce Donald, a professor of computer science and biochemistry at Duke University, in a press release.

The research (“Intracellular Neural Recording with Pure Carbon Nanotube Probes”), which was published in the journal PLoS ONE, overcame the shortcomings (literally) of other attempts to use carbon nanotubes (CNTs) as neuron probes. Previously, CNTs have proven to be too short or too thick for the job. But the Duke team was able to make their CNT probe one millimeter long (quite long for CNTs) and capable of monitoring the electrical signals between neurons more precisely than the glass or metallic electrodes that are typically used.

The researchers were able to achieve these unique CNT characteristics with a specially devised technique. They accumulated carbon nanotubes at the tip of a tungsten wire until the tubes took the shape of a needle-like probe. Next, they coated the probe with an insulating material and then removed the insulating material with a focused ion beam. This process of applying, then removing the insulating material gave the probe an extremely fine point.

"The results are a good proof of principle that carbon nanotubes could be used for studying signals from individual nerve cells," said Duke neurobiologist Richard Mooney, a study co-author, in press release. "If the technology continues to develop, it could be quite helpful for studying the brain."

While the researchers concede that more research needs to be done to improve the electrical recording capabilities of the probes—even as improvements are made to their geometry and the insulating layers—the Duke team has applied for a patent on the probe. The researchers expect that the technology could not only prove useful for mapping the brain but for creating brain-computer interfaces.

Photo: Inho Yoon and Bruce Donald, Duke


Graphene Comes to the Rescue of Molecular Electronics

Molecular electronics are a long-proposed—and sometimes forgotten—aim for electronics. The field promises a time when the basic building blocks of electronics are individual molecules. But a reliable method for testing these molecular components has remained elusive.

Now a joint research team comprising chemists and physicists from the Department of Chemistry Nano-Science Center at the University of Copenhagen and the Chinese Academy of Sciences in Beijing has developed a graphene-based chip whose initial application could be testing the molecular chips researchers envision.

The research (“Ultrathin Reduced Graphene Oxide Films as Transparent Top-Contacts for Light Switchable Solid-State Molecular Junctions”), which was published in the Wiley journal Advanced Materials, claims to be the first time that a transistor composed of just one molecular monolayer functioned on a chip.

While Kasper Nørgaard, an associate professor of chemistry at the University of Copenhagen, believes that the first applications for the graphene-based chip will be in testing future molecular electronics, the chip itself represents a first step towards integrated molecular circuits.

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Spot Welding Graphene Transistors on the Atomic Scale

With researchers still struggling to open a band gap in graphene at room temperature sufficient for transistor applications, it’s sometimes good to remember what makes graphene such an appealing material that they think it's worth the struggle—after all, an awful lot of effort has gone into engineering a band gap into a material that intrinsically doesn’t have one. High among those benefits is the promise of high electron mobility and simpler chemical doping techniques—with a far easier path to interconnection than its cousin, carbon nanotubes.

To buoy hopes of graphene in transistor applications, researchers at Aalto University in Finland and Utrecht University in the Netherlands have demonstrated the ability to create single atom contacts between gold and graphene nanoribbons.

The research ("Suppression of electron–vibron coupling in graphene nanoribbons contacted via a single atom"), which was published in the journal Nature Communications, showed that contacts between graphene and gold could be established without significantly modifying the very electrical properties of graphene’s honeycomb lattice that make it so attractive in the first place.

The process starts with an atomic-scale mapping of the graphene using atomic-force microscopy (AFM) and a scanning tunneling microscope (STM). Then a chemical bond is achieved by sending voltage pulses from the tip of a STM to create single bonds to the graphene nanoribbons at precisely determined locations. The pulse from the STM removes one hydrogen atom from the end of the graphene nanoribbon, initiating the bond formation.

"The edges of the chemically synthesized ribbons that we use are hydrogen terminated just as you would have in a molecule (e.g., pentacene)," Professor Peter Liljeroth, who heads the Atomic Scale Physics group at Aalto University, explained to me in an e-mail. "We can use bias voltage pulses from the STM tip to knock off the hydrogen atoms one-by-one. When you remove a single hydrogen, you form what a chemist would call a radical and a physicist would call a dangling bond: the carbon atom without the hydrogen has an unpaired electron that would like to form a bond with something. It does this with one of the atoms of the underlying gold substrate. So we remove the hydrogen, the carbon atom becomes more reactive and forms a bond spontaneously with one of the gold atoms."

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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